I ran a TN0106 in my high band QDX for several months and had no issues. I had better output on 10 meters than with the BS170 but nothing dramatically better. I also ran the VN0606 in my low band unit at the same time and also had no issues and performance was good. Those tests were with otherwise unmodified QDXs. Now I am running with BS170s again but with a 1N4148 commutating diode across L14. On my scope I could see that the spike at end-of-tx was eliminated with this diode.

My original problem was that I fried three sets of BS170 in a few weeks. Every thing seems OK with the diode or when using the TN0106 or VN0606.

What I could see on my scope with the Bs170 as originally designed was a 70 volt spike, presumably discharging L14 at EOT, that was chopped off at 70 volts apparently due to avalanche. It had a wide flat top. With the TN0106 and VN0606 the spike was up to 120 volts but with a narrower flat top. Also avalanche but at a higher voltage level.?

It appears that the TN0106 and VN0606 both avalanche at a level more like a 100 volt transistor and I suspect that the manufacturer (same for both devices) is selecting the 60 volt devices from runs of the 100 volt devices. Not so for the BS170 which avalanche at a level consistent with a 60 volt device.

Anyway, I think that the diode across? L14 is a better solution than using the TN0106 or VN0606. Yes, both methods work, but why stress the transistors at all? It seems to me that you have more margin with the diode. Likewise for using Zeners, again it probably works but you are adding two parts instead of one and adding parts on the drains of your PA devices. Also you are discharging L14 through the output transformer winding which seems sub-optimal to me.?

I have an issue in my antenna system where the SWR goes up when it rains. It has been dry recently but we had rain again a couple of nights ago and I made it a point to get on 20 meters where the SWR was up to 2.5 to 1 (antenna impedance here goes low when it rains btw) and I ran for an hour with no ill effects so my confidence in the diode approach is building.

Just my perspective on this issue. Based on my experiences thus far with QDX and my 48 years as an EE, I would add the diode. It is cheap protection and is situated where it can't really hurt anything. During transmission, the inductor current is basically DC and so there is no risk of creating spurs. It is only at the end where the current stops that the diode does anything.

73,

Tony

AD0VC